The steel sheet generally used as good corrosion resistance plated steel sheet is hot dip galvanized steel sheet. This hot dip galvanized steel sheet can usually be produced by degreasing steel sheet, then preheating it in a non-oxidizing furnace, reduction annealing it in a reduction furnace to clean the surface and secure quality, dipping it in a hot dip zinc bath, and controlling the amount of deposition. This can provide excellent shapeability, corrosion resistance, plating adhesion, etc., so is widely being used for automobiles, building material applications, etc.
In particular, recently, in the automobile sector, to both secure the function of protecting the passengers at the time of impact and reducing the weight for improving the fuel economy, plated steel sheet is being required to be made of a higher strength.
To increase the strength of steel sheet without detracting from the workability, it can be effective to add elements such as Si, Mn, and P. Among these, Si is particularly easily oxidized even compared with Fe, so it is known that if plating steel sheet containing Si under ordinary hot dip galvanization conditions, during the annealing process, the Si in the steel may concentrate at the surface and cause non-plating defects and a drop in plating adhesion. Further, the addition of these elements can delay the alloying, and thus the higher temperature, the longer the time of alloying required compared with mild steel. This higher temperature, longer time alloying generally causes the austenite remaining in the steel sheet to transform to pearlite, and can lower the workability. Thus, as a result, the effects of the added elements are cancelled out.
As technology for suppressing non-plating defects in steel sheet containing Si, Japanese Patent Publication (A) No. 55-122865 describes the method of oxidizing steel so that the surface is formed with an oxide film of a thickness of 400 to 10000 Å, then annealing and plating it in an atmosphere containing hydrogen. However, in this document, the adjustment of the reduction time of the iron oxide film is difficult in practice. If the reduction time is too long, surface concentration of Si is likely caused, while if too short, iron oxide film generally remains on the steel surface. Thus, there may be a problem that this does not completely eliminate plating defects and the problem that if the iron oxide film on the surface becomes too thick, the peeled off oxides may likely stick to the rolls and cause flaws in the appearance.
To deal with these problems, Japanese Patent Publication (A) No. 2001-323355 and Japanese Patent Publication (A) No. 2003-105516 has been provided which describe a method of production preventing surface concentration of Si by oxidizing the steel sheet surface, then reducing it in a reducing furnace controlled in atmosphere. Further, Japanese Patent Publication (A) No. 2001-295018 describes the use of Si-containing high strength hot dip galvanized steel sheet excellent in corrosion resistance comprised of steel sheet having an Si content of 0.2 to 2.0 mass % formed on its surface with a hot dip Zn—Al—Mg plating layer comprised of Al: 2 to 19 mass %, Mg: 1 to 10 mass %, and the balance of Zn and unavoidable impurities and further. Further, for example, Japanese Patent Publication (A) No. 2004-323970 describes a high strength hot dip galvanized steel sheet excellent in plateability comprising steel sheet having an Si content of 0.2 to 3.0 mass % containing inside its surface oxide particles of one or more of Si oxides, Mn oxides, or Si and Mn composite oxides.
Further, Japanese Patent Publication (A) Nos. 56-33463 and 57-79160 describe methods for suppressing non-plating defects by pre-plating a steel sheet surface with Cr, Ni, Fe, etc. Further, Japanese Patent Publication (A) No. 2002-161315 describes a method for forming an internal oxide layer directly under the surface of steel sheet on a continuous annealing line, removing the simultaneously formed surface oxides by pickling, then plating the sheet on a continuous hot dip galvanization line.